As illustrated in Japanese Patent Publication No. 2002-124291, and as illustrated in FIGS. 16 and 17, a fuel cell, for example, a Polymer Electrolyte Fuel Cell (PEFC) apparatus 10 includes a layering structure of a Membrane-Electrode Assembly (MEA) and a separator 18. The layering direction may be in any direction.
The MEA includes an electrolyte membrane 11 made from an ion exchange membrane and a pair of electrodes which includes an anode 14 disposed on one side of the electrolyte membrane and a cathode 17 disposed on the other side of the electrolyte membrane. A diffusion layer 13 may be disposed between the anode and the separator 18, and a diffusion layer 16 may be disposed between the cathode and the separator 18.
A first separator 18 disposed on one side of the MEA has a fuel gas passage 27 formed therein for supplying fuel gas (hydrogen) to the anode 14, and a second separator 18 disposed on the other side of the MEA has an oxidant gas passage 28 for supplying oxidant gas (oxygen, usually, air) to the cathode 17. The first and second separators 18 have a coolant passage 26 on opposite sides of the fuel gas passage 27 and the oxidant gas passage 28. In order to seal the fluid passages 26, 27 and 28 to each other, a rubber gasket 32 is disposed between adjacent fuel cells and an adhesive seal 33 is provided between the separators 18 disposed on opposite sides of the MEA of each fuel cell.
At least one (three at most) fuel cell 19 constructs a module. A number of modules are piled, and electrical terminals 20, electrical insulators 21, and end plates 22 are disposed at opposite end of the pile of modules to construct a stack of fuel cells (a fuel cell stack) 23. After tightening the stack of fuel cells between the end plates 22 in the fuel cell stacking direction, the end plates 22 are coupled to a fastening member 24 (for example, a tension plate) extending in the fuel cell stacking direction outside the pile of modules by bolts or nuts 25.
In the PEFC, hydrogen changes to positively charged hydrogen ions (i.e., protons) and electrons at the anode 14. The hydrogen ions move through the electrolyte membrane 11 to the cathode 17 where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of an adjacent MEA and move to the cathode 17 of the instant MEA through a separator, or which are generated at an anode of a fuel cell located at one end of the fuel cell stack and move to the cathode 17 of the instant fuel cell located the other end of the fuel cell stack through an external electrical circuit) to form water as follows:
At the anode: H2→2H++2e−
At the cathode: 2H++2e−+(1/2)O2→H2O
In the conventional fuel cell stack, the modules are held as follows:
A spring 34, a tilting portion 35 and an adjusting screw 36 are provided at one end of the stack of fuel cells 23. Each module of the stack 23 is held in the fuel cell stacking direction by a constant force generated by the spring 34 and is held in a direction perpendicular to the fuel cell stacking direction by a frictional force defined as a spring force multiplied by a coefficient of friction.
In order to hold the modules more securely, it will be conceived to fill a space between the side surface of the pile of modules and the tension plate with an external restraining member to restrain the pile of modules from outside.
However, the following problems with the above fuel cell stack exist:
i) When an impact force with an acceleration (α) of several to twenty G (G: acceleration of gravity) acts on the stack of fuel cells of mass M in the direction perpendicular to the fuel cell stacking direction, a shear force is generated near the end of the stack of fuel cells. When the shear force exceeds the spring force multiplied by a coefficient of friction, slippage occurs between the modules near the end of the stack of fuel cells accompanied by disassembly of the stack of fuel cells.
ii) In the case where the modules are held from the outside by the external restraining member, when the MEA and the diffusion layers of the fuel cell cause a creep receiving the spring force and the fuel cell near the end of the fuel cell stack moves in the fuel cell stacking direction relative to the external restraining member, the fuel cell interferes with the external restraining member, which is also damaged. If the spring force is made small in order to decrease a creep amount, it is difficult to ensure a necessary contact pressure between the fuel cells.
A first problem solved by the present invention is the problem of disassembly of the stack near the end of the stack which occurs when an impact force with an acceleration in a direction perpendicular to the fuel cell stacking direction acts on the stack.
A second problem solved by the present invention is the problem of damage to the fuel cell which may occur in the stack having an external restraining member when the fuel cell near the end of the stack moves relative to the external restraining member due to creep of the MEA and the diffusion layer.